Prop for use in underground mining or tunnel construction

ABSTRACT

An extending prop for underground and tunnel construction, comprising, an outer tube having a closed bottom end, an inner tube telescopically slidable in the outer tube and having a closed top end, and a mechanical locking device adapted to fix the relative positions of the inner and outer tubes. A hydraulically actuated flying piston is arranged in the outer tube and is separated from the inner tube. The piston is adapted to extend the inner tube upon actuation by hydraulic pressure. The piston divides the prop into a bottom chamber enclosed by the outer tube and a top chamber enclosed by the inner tube. A first filling connection is arranged on the outer tube for filling the bottom chamber with a pressure medium for actuating the piston. A second filling connection is arranged on the inner tube for filling the top chamber. The prop is fillable with construction material through the second filling connection over its entire length by displacement of the piston toward the bottom end of the the outer tube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an extending prop for underground mining ortunnel construction, consisting of two telescopic tubes which are closedat their free ends and designed to be braced against the rock by meansof a hydraulically operating prop-extending device, with provision beingmade between said tubes for a locking device, by which the inner tubecan be mechanically fixed on the outer tube at different lengths ofextension, whereby the interior spaces of the extending prop can befilled with construction material while the extension load is beingmaintained.

2. The Prior Art

Such extending props filled with construction material are nowfrequently used instead of the wooden props or individual hydraulicprops employed in the past. The advantage is that such props, giventheir lightweight construction, are capable of generating very highforces of support. Since the tubes substantially only serve astension-resistant sleeves for the largely compression-resistantconstruction material, their steel construction can be designed withconsequent application of the design principles of lightweightengineering. Simple welded tubes made of inexpensive structural steelcan be used because stressing of the tubes is not critical. The weightof the props is very light as long as the tubes are still empty, so thatthey can be easily transported and handled at the site of installation.Because of the lightweight design and inexpensive manufacture madepossible by such a design, said props, as opposed to individualhydraulic props, can be used as so-called "lost expansion" which, afterit has been used, for example in the brace/gallery transition zone, canbe left standing without problems in the packing space. Such lostexpansion within the zone of the edge of the gallery, furthermore,offers advantages in view of the future stability of the gallery.

Such a prop filled with construction material is known, for example fromDE-PS 41 15 209. Before the construction material is filled, said knownprop uses for bracing a plurality of hydraulic clamping cylinders, whichare attached to the outer and inner tubes from the outside fordisplacing the inner tube against the outer tube. In the extendedposition, the inner tube is locked on the outer tube in the givenposition of extension by a locking device. The hydraulic clampingcylinders then can be removed. Subsequently, the prop so braced againstthe rock is filled with construction material.

The known prop of said type has drawbacks in various respects. On theone hand, handling of the hydraulic clamping cylinders is extremelycomplicated and labor-intensive. Each of said heavy clamping cylindershas to be lifted by hand, attached to the prop, and then removed againfrom the latter after bracing has been completed. On the other hand,possibilities have to be available for connecting the cylinders with ahigh-pressure hydraulic system, for example a connection with thebracing hydraulics. Furthermore, the flow of forces obtained isunfavorable because of the lateral attachment of the clamping cylinders.In addition, the extent of telescoping is relatively limited.

Finally, another drawback is that the prop can absorb only low forcesuntil it is filled with construction material, because the mechanicallocking device always first requires a certain amount of insertion ofthe inner tube against the outer tube before it is capable of absorbingnotable forces. The known props, therefore, are not sufficiently bearingin the early stage and require a relatively large amount of convergencebefore developing their full supporting force.

The older DE 43 38 830 Cl--which was not published at an earlierdate--discloses a similar extending prop, in which the inner end of theinner tube is designed closed, forming in this way a piston that isguided sealed in the outer tube. The interior space of the extendingprop is divided by such cylinder in two chambers, which both can befilled with construction material, for example with compressible foamedconcrete. With said extending prop, however, the piston does not servethe purpose of setting the prop with high setting load, but forobtaining in cooperation with the compressible filling material adefined yieldingness of the prop. For extension, said known prop uses aprop extension mechanics, which is located at the insertion end of theouter tube and fixable on the inner tube with vertical adjustability. Nohigh extension loads can be achieved with said mechanical prop extensiondevice. Furthermore, it is not possible to achieve high extension loadsby filling the interior spaces of the prop with expanding foam materialbecause the latter is compressible and, consequently, not capable ofgenerating any high extension load.

An extending prop is known also from WO 94/27029, which consists of twotelescopic tubes which are closed at the ends facing away from eachother. The tubes can be hydraulically braced against the rock and fixedon each other with a locking device at different lengths of extension.The outer tube and the inner tube enclose a chamber, which can be actedupon via a filling connection by pressure medium, whereby the inner tubeis extended from the outer tube, i.e., the extending prop is set.However, the interior spaces of this known prop cannot be filled withconstruction material, which means that the advantages of this specialtechnology are not exploited.

The problem of the invention is to create an extending prop of the typespecified above, which can be handled in a simple way, extendedhydraulically with high extension load; which has immediate or at leastearly bearing characteristics, and which can be filled with constructionmaterial over its entire length.

SUMMARY OF THE INVENTION

The object of the invention is an extending prop for underground miningand tunnel construction, consisting of an inner tube and an outer tube,such tubes being designed closed at their free ends and telescopic andbeing fixable against each other at different lengths of extension bymeans of a mechanical locking device, whereby a piston acted uponhydraulically by extension pressure is arranged in the outer tube, suchpiston acting upon the inner tube in the sense of extension and beingdesigned as a flying structural component separated from the inner tubeand dividing the interior space of the extending prop in a first chamber(bottom chamber) located beneath the piston, and a second chamber (topchamber) located above the piston, whereby a first filling connection isassociated with the first chamber (bottom chamber), via which saidchamber can be acted upon by a pressure medium under extension pressure,and a second filling connection is associated with the second chamber(top chamber), via which the extending prop can be filled over itsentire length with construction material by displacement of the flyingpiston up to the end of the outer tube.

The extending prop according to the invention has the advantage, firstof all, that extension and bracing against the rock are extremely simpleoperations. For said purposes, only the bottom chamber of the extendingprop is acted upon by a suitable hydraulic pressure medium. Primarilywater from the low-pressure water pipeline--which is availableunderground, as a rule--is suitable for said purpose. Such pipelineusually carries water under a pressure of 40 bar, which, in view of thelarge diameter of the outer tube, suffices for generating a substantialextension force. The operations taking place during the extensionprocess are as simple as with individual hydraulic props. Especiallycomplicated manipulation of separate extending cylinders is dispensedwith. Like an individual hydraulic prop, the extending prop so set isdistinctly early bearing and thus puts up considerable supporting forceagainst the starting convergence from the start. After a certain amountof convergence has occurred, the mechanical locking device develops itsfull supporting force, so that the pressure medium loading the pistoncan be discharged and the prop can be filled with construction materialvia the second filling connection. In this process, the flying piston ispushed back to any desired extent, if necessary to the bottom of theouter tube. During said filling operation, the inner tube is fixed onthe outer tube by means of the mechanical locking device. By filling theextending prop with construction material over any desired length, it ispossible to influence the support behavior of the prop within widelimits without requiring any additional technical expenditure.

According to a particularly advantageous embodiment of the extendingprop according to the invention, provision is made that the mechanicallocking device consists of two telescopic ring wedge elements arrangedbetween the lower end of the inner tube and the piston. One of said twoelements is radially spreadable apart when the two elements are pushedone into the other, plastically expanding the outer tube. Such design ofthe mechanical locking device results in an early bearing and highlyload-carrying mechanical connection between the inner and the outertube. A special advantage of such mechanical connection is that theinner tube remains displaceable against the outer tube over a relativelygreat distance while maintaining a very high extension load. The outertube is plastically widened in this connection across the length of thesink-in path. Such plastic widening is visible on the extending propfrom the outside and tells the miner that a certain amount ofconvergence has already occurred on the respective prop.

So as to avoid that the variation of the locking device discussed aboveis activated too early when the prop is extended, for example becauseresistances have to be overcome when the prop is pushed out, provisionis made, furthermore, that the wedge surfaces of the ring wedge elementsand/or counter ring wedge elements are fitted with projections that canbe sheared off at a preset extension load. Such projections releasedisplacement of the wedge surfaces against each other only when thepreset extension load is exceeded.

Alternatively, the locking device can be designed in the form of anexpanding spring ring arranged on the lower end of the inner tube, suchspring ring being elastically pretensioned in the radial direction anddesigned in such a way that it permits extension movement of the innertube relative to the outer tube, but expands in the radial direction inthe presence of a retraction movement, in a way such that a frictiongrip and/or positively locked connection connection with the outer tubeis produced.

Furthermore, the locking device may have a ring wedge fixable on theouter circumference of the inner tube, such ring wedge being insertablein the outer tube from the top, expanding the latter. Such a lockingdevice is yielding across a very great distance of movement as well,while high supporting force is maintained, and permits providing theextending prop with a suitable support characteristic.

With the last-mentioned design of the locking device, the ring wedge isusefully fitted on its inner circumference with means increasing thefriction between the ring wedge and the inner tube. In this way, thering wedge locks itself on the inner tube in the axial direction, sothat yieldingness of the locking device is primarily supplied by plasticexpansion of the outer tube.

Furthermore, with said embodiment of the locking device, provision ismade for a ring made of elastic, pretensioned material, such ringresting against the top side of the ring wedge and in turn beingsupported on the upper end of the outer tube and pushing the ring wedgedownwardly in the axial direction. This assures that when convergencestarts, the ring wedge immediately absorbs the expansion of the outertube. This means that the extending prop develops the full supportingforce within the zone of the locking device already after a relativelyshort distance of sink-in.

The piston used for the prop according to the invention usefullyconsists of elastically and/or pladtically yielding material, inparticular plastic, and has an annular lip seal snugly fitted againstthe wall of the outer tube, and is mounted on the bottom of a pot-like,stiff guiding element. Such a piston can be manufactured at very lowcost and is adequately resistant to weat for the one-time use of theextending prop.

A pressure-limiting valve can be usefully associated with the bottomchamber. Such pressure-limiting valve--which is basically known fromhydraulic props--offers the advantage that, in combination with theflying piston, the prop can be filled, if necessary, with constructionmaterial over the entire length while the pressure prevailing in thebottom chamber is being maintained. However, this requires aconstruction material pump for supplying the construction material to befed into the top chamber at a pressure slightly greater than the openingpressure of the pressure-limiting valve.

Special benefits are gained if a bursting disk serving as thepressure-limiting means is associated with the bottom chamber. Suchbursting disk, like the pressure-limiting valve, responds when a definedpressure is exceeded. Such pressure occurs after convergence hasprogressed accordingly and the extending prop is loaded to an extentsuch that the mechanical locking device develops its full supportingforce. The broken bursting disk tells the miner on site that he can nowfill this extending prop with construction material.

In order to further favorably influence the support properties of theextending prop according to the invention, elastic, compressibleelements can be arranged in the bottom chamber, particularly elements inthe form of air-filled hollow bodies, or foam bodies. In this way, thebottom chamber is provided with exactly preselectable properties untilthe full supporting force clicks in. The support properties of theextending prop can be changed in any desired way by filling such bodiesto a greater or lesser extent.

For the last-mentioned purpose, the extending prop can be fitted, ifnecessary, with axially compressible spring elements arranged on thefoot or head side.

So as to limit the amount of extension of the prop according to theinvention, the wall of the outer tube is provided with bores spaced fromthe top end of the outer tube, such bores serving as stroke-limitingmeans. When the piston travels across said bores, the latter connect thebottom chamber with the atmosphere. Such limitation of the stroke isachieved in a particularly advantageous way without stop means, whichwould require much more complicated working of the tubes.

Finally, provision is made for fitting the walls of the inner and outertubes within the zone of overlap with dewatering bores, which supplementor support the dewatering device located in the head of the prop. Thedewatering bores permit water to exit from the filled constructionmaterial. Such withdrawal of excess water is important, so that theconstruction material can form a supporting column and, if necessary,set the latter.

The dewatering bores of the outer tube are usefully covered by a fabricthat is permeable to water and, at the same time, retains theconstruction material. In this way, the annular space between the innerand outer tubes is still filled with construction material, whereasconstruction material is at the same time prevented from exiting fromthe extending prop via the dewatering bores.

Finally, according to an advantageous further development of theextending prop according to the invention, provision is made that theouter tube and the inner tube are arranged in the interior of ajacketing consisting of two tubes, which are plugged one into the otherand telescopically guided in one another, such jacketing having a largerdiameter than the prop across its entire length, whereby the annularspace between the extending prop and the jacketing can be filled withconstruction material. It is possible in this way to further develop theprop according to the invention into a prop with very high bearingcapacity, such prop being capable of absorbing extremely high loads.Such an extending prop with such jacketing can be used, for example forsupporting bridge panels in gallery intersections or the like. Thejacketing may have practically any desired diameter, so that supportingforces of any desired magnitude can be obtained. Due to the fact thatthe prop of the invention is arranged in the interior of the jacketing,this load-bearing prop, too, can be braced with high force betweenhanging and lying rock.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplified embodiments of the invention are explained in greater detailin the following by reference to the drawings, in which:

FIG. 1 shows a schematic view of the extending prop of the invention bya longitudinal section, without pressure medium and constructionmaterial;

FIG. 2 shows the extending prop according to FIG. 1 in the course of thesetting process.

FIG. 3 shows the extending prop according to FIG. 1 as convergencestarts.

FIG. 4 shows the extending prop according to FIG. 1 prior to fillingwith construction material.

FIG. 5 shows the extending prop according to FIG. 1 after the fillingwith construction material.

FIG. 6 shows an extending prop according to the invention with anexpanding spring ring as locking device.

FIGS. 7, 8 and 9 show an extending prop according to the invention withanother design of the locking device.

FIG. 10 shows a further development of the extending prop shown in FIGS.7, 8 and 9.

FIG. 11 shows a representation of a detail of the piston.

FIG. 12 shows an extending prop according to the invention with abursting disk on the bottom chamber.

FIG. 13 shows an extending prop according to the invention with anelastic head part.

FIG. 14 shows an extending prop according to the invention withcompressible hollow bodies in the bottom chamber.

FIG. 15 shows an extending prop according to the invention withlimitation of the stroke and dewatering bores.

FIG. 16 shows detail representations of a locking device with ringwedges.

FIG. 17 shows an extending prop according to the invention with aspecial design of the piston; and

FIG. 18 shows an extending prop according to the invention with ajacketing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the extending prop in its totality is denoted by referencenumeral 1. It consists of an outer tube 2, which is closed at the bottomand open at the top, in which an inner tube 3 is extendibly guided, saidtube being open at the bottom and closed at the top. A piston 4 locatedat the lower end of inner tube 3 is sealingly guided in outer tube 2 andserves for pushing inner tube 3 out. A first chamber (bottom chamber 5)is locared beneath piston 4 in outer tube 2, said chamber being actedupon via a filling connection 6 by a suitable pressure medium, forexample pressure water.

A second chamber (top chamber 7) is located above piston 4, said chamberbeing substantially formed by the inner tube 3 and having a fillingconnection 8 at the upper end.

Inner tube 3 is fixable on outer tube 2 by means of a locking device 9.Said locking device 9 has a ring wedge 10 fixable on the outercircumference of inner tube 3, said ring wedge being insertable in outertube 2 from the top, expanding outer tube 2.

Piston 4 is designed as a flying piston, i.e., it is a structuralelements separated from inner tube 3, resting supportingly against innertube 3, supporting the latter from the bottom.

Setting and filling of the extending prop shown in FIG. 1 is explainedin the following in greater detail. As shown in FIG. 2, extending prop 1is first between hanging lying rock. For said purpose, bottom chamber 5is acted upon via filling connection 6 by a pressure medium suitable forthe setting process, e.g. pressure water from a low-pressure waterpipeline. Piston 4 is forced upwardly in this way, pushing inner tube 3against the hanging material with relatively high setting force.

Filling connection 6 is subsequently shut off. When convergence starts,inner tube 3 is pushed downwardly with great force, so that outer tube 2is slightly deformed elastically, i.e., it becomes slightly thickerunder the the influence of the pressure prevailing in bottom chamber 5.As the depth of penetration increases, ring wedge 10 penetrates the topend of the outer tube and elastically expands the latter. Locking device9 is gradually loaded in this way, to an extent such until it alone iscapable of absorbing the required force of support. Bottom chamber 5 issubsequently drained. In this condition, the supporting force is solelygenerated by locking device 9.

For increasing the supporting force, the extending prop is now filledwith construction material from the top via filling connection 8 (FIG.5). In the course of filling with construction material from the top,the construction material forces piston 4 into the outer tube 2 down tothe bottom of the latter, so that the extending prop is filled withconstruction material over its entire length. If only part of the lengthof the extending prop is to be filled with construction material inorder to obtain a certain bearing behavior, the filling operation can bediscontined at any time.

Filling with construction material can be carried out at any desiredtime as soon as the locking device 9 is bearing sufficiently. In thisway, the bearing or supporting behavior of the extending prop of theinvention can be adapted to the rock pressure and occurring convergencesat any time. In any case, the miner has numerous possibilities still onsite to influence the bearing behavior of the extending prop of theinvention in said sense.

The exemplified embodiment of FIG. 6 largely corresponds with theexemplified embodiment according to FIG. 1. However, in the former, anexpanding spring ring 11 is arranged at the lower end of inner tube 3,said spring ring being elastically pretensioned in the radial directionand resting with its elastic tongues against the inner wall of outertube 2. The spring tongues are arranged in such a way that the innertube together with the expanding spring ring 11 can be displacedupwardly, whereas displacement downwardly is not permitted by suchrongues. When displaced downwardly, the tongues of the expanding springring spread in the radial direction in such a way that a friction gripor positive connection with outer tube 2 is produced.

In the embodiment of the locking device shown in FIGS. 7, 8 and 9, innertube 2 is fitted at its lower end with a radially expandable ring wedgeelement 12, into which a counter ring wedge element 13 located abovepiston 4 and supported on the latter is insertable from the bottom. Whencounter ring wedge element 13 is pushed into the radially spreadablering wedge element 12 from the bottom, the latter expands, plasticallyexpanding the outer tube 2, so that an extremely supportive connectionwith immediately bearing properties is obtained between inner tube 3 andouter tube 2.

With the embodiment according to FIG. 10, radially inwardly protrudingprojections 12a, which can be sheared off, are located on the wedgesurfaces of ring wedge element 12. Said projections first have to besheared off by the counter ring wedge element 13 before counter ringwedge element 13 can be pushed into ring wedge element 12. The size ofthe cuttable cross sections of projections 12a is selected in such a waythat insertion of counter ring wedge element 13 into ring wedge element12 can take place only when a predetermined, relatively high settingload is exceeded. For the same purpose, if necessary, provision can bemade for corresponding cuttable projections on the wedge surfaces ofcounter ring wedge element 13 as well.

FIG. 11 shows a representation of a detail of piston 4. Said pistonconsists of elastically and/or plastically yielding material,particularly plastic, and has a ring lip 4a snugly fitting against thewall of the outer tube. Furthermore, piston 4 is mounted on the bottomof a pot-shaped, rigid guiding element 4b which, for example, is made ofsteel. Said guiding element 4b provides the piston with the requiredguidance and rigidity, whereas lip seals 4a provided for tightness. Apiston so designed is, overall, inexpensive to manufacture and,therefore, especially suitable for one-time use.

FIG. 12 shows an exemplified embodiment of the extending prop accordingto the invention, in which a bursting disk 14 serving as pressurelimiter is associated with bottom chamber 5. Said bursting disk respondswhen loading of the extending prop has progressed to a point wherelocking device 9 is adequately bearing.

In the exemplified embodiment according to FIG. 13, inner tube 3 isfitted on the head side with the axially compressible spring elements15. If necessary, such spring elements 15 can be arranged also on outertube 2 on the foot side.

FIG. 14 shows that elastically compressible hollow bodies particularlyin the form of air-filled hollow bodies 14 or foam bodies can bearranged, if necessary, in bottom chamber 5. Such elasticallycompressible hollow bodies 14 effect additional elastic yieldingness ofthe extending prop.

FIG. 15 shows that provision is made in the wall of outer tube 2 for thebores 18, such bores being arranged spaced from the top end of the outertube and serving as stroke-limiting means. Said bores connect bottomchamber 5 with the atmosphere when the piston travels across said bores.As soon as said connection is made, piston 4, of course, no longer canbe moved upwardly further.

Furthermore, FIG. 15 shows the dewatering bores 19 in inner tube 3, and20 in outer tube 2, such bores being located within the zone of overlapof inner tube 3 and outer tube 2. The dewatering bores 20 of outer tube2 are covered on the outside by a fabric 21, which is permeable towater, but impermeable to construction material. In addition, theextending prop has, of course, also the usual dewatering device in thehead of the prop.

FIG. 16 shows a modified embodiment of locking device 9. Here, ringwedge 10 is fitted on its inner side with the sharp projections 10a,which engage the outer wall of inner tube 3. Furthermore, a ring 22 madeof elastically pretensioned material is arranged above ring wedge 10,said ring itself being supported on the top end of outer tube 2 andpressing ring wedge 10 downwardly in the axial direction.

In the exemplified embodiment according to FIG. 17, piston 4 has adownwardly protruding elastic projection 4c which, after the extendingprop has been filled with construction material, abuts the bottom ofouter tube 2, providing for elastic support behavior as well.

In the exemplified embodiment according to FIG. 18, the extending propis arranged in the interior of a jacketing consisting of two tubes 23,24, which are plugged one into the other and guided in the way of atelescope. Said jacketing has a larger diameter than extending prop 1across its entire length, whereby the annular space between extendingprop 2, 3 and jacketing 23, 24 is fillable with construction materialvia a filling connection 25 arranged on top. As explained above,extending prop 2, 3 arranged in the interior of jacketing 23, 24 can beset with high setting force by hydraulically acting upon piston 4; itcan be fixed mechanically with locking device 9, and filled over itstotal length with construction material by way of filling connection 8.Such extending prop, in combination with the jacketing, has extremelyhigh bearing capacity.

I claim:
 1. An extending prop for underground and tunnel construction,comprising:an outer tube having a closed bottom end; an inner tubetelescopically slidable in said outer tube and having a closed top end;a mechanical locking device adapted to fix the relative positions ofsaid inner and outer tubes; a hydraulically actuated flying pistonarranged in said outer tube and separated from said inner tube, saidpiston adapted to extend said inner tube, wherein said piston dividesthe prop into a bottom chamber enclosed by said outer tube and a topchamber enclosed by said inner tube; a first filling connection arrangedon said outer tube for filling said bottom chamber with a pressuremedium; and a second filling connection arranged on said inner tube forfilling said top chamber, wherein the prop is fillable with constructionmaterial through said second filling connection along its entire lengthby displacement of said piston toward the bottom end of the the outertube.
 2. Extending prop according to claim 1, characterized in that themechanical locking device consists of two telescopable ring wedgeelements (12, 13) arranged between the bottom end of the inner tube (3)and the piston (4), one (12) of said element being radially expandableduring telescoping with plastic widening of the outer tube (2). 3.Extending prop according to claim 2, characterized in that the wedgesurfaces of the ring wedge element (12) and/or the counter ring wedgeelement (13) is/are provided with projections (12a) cuttable at apredetermined setting load, said projections releasing displacement ofthe wedge surfaces against each other only when the predeterminedsetting load is exceeded.
 4. Extending prop according to claim 1,characterized in that the locking device has a ring wedge (10) fixableon the outer circumference of the inner tube (3), said ring wedge beinginsertable in the outer tube (2) from the top with plastic widening ofof the outer tube (2).
 5. Extending prop according to claim 4,characterized in that the ring wedge (10) is fitted on its innercircumference with means (11a) increasing the friction between the ringwedge (10) and the inner tube (3).
 6. Extending prop according to claim2, characterized in that a ring (22) made of elastically pretensionedmaterial rests against the top side of the ring wedge (10), said ringitself being supported on the top end of the outer tube (2) and pressingthe ring wedge (10) downwardly in the axial direction.
 7. Extending propaccording to claim 1, characterized in that the locking device isdesigned in the form of an expanding spring ring (11) arranged at thelower end of the inner tube, said spring ring being elasticallypretensioned in the radial direction and designed in such a way that itpermits a movement of extension of the inner tube (3) relative to theouter tube (2), but spreads in the radial direction during a movement ofretraction in such a way that a friction grip and/or positive contactwith the outer tube (2) is established.
 8. Extending prop according toclaim 1, characterized in that the piston (4) consists of elasticallyand/or plastically yielding material, in particular plastic; that it hasan annular lip seal (4a) snugly fitting against the wall of the outertube (2); and that it is mounted on the bottom of a pot-shaped, rigidguiding part (4b).
 9. Extending prop according to claim 1, characterizedin that a pressure-limiting valve is associated with the bottom chamber(5).
 10. Extending prop according to claim 1, characterized in that abursting disk (12) serving as pressure limiter is associated with thebottom chamber (5).
 11. Extending prop according to claim 1,characterized in that plastically compressible bodies, in particular inthe form of air-filled hollow bodies (14) or foam bodies are arranged inthe bottom chamber (5).
 12. Extending prop according to claim 1 or anyone of the following claims, characterized in that it is fitted withaxially compressible spring elements (15) on the foot or head side. 13.Extending prop according to claim 1 or any one of the following claims,characterized in that the wall of the outer tube (2) is provided withbores (18) spaced from the upper end of the outer tube (2) and servingas stroke-limiting means, said bores connecting the bottom chamber (5)with the atmosphere when passed over by the piston (4).
 14. Extendingprop according to claim 1 or any one of the following claims,characterized in that the walls of the inner tube (3) and outer tube (2)are provided with dewatering bores (19, 20) within the zone ofoverlapping.
 15. Extending prop according to claim 14, characterized inthat the dewatering bores (20) of the outer tube (2) are covered by afabric (21), such fabric being permeable to water but retaining theconstruction material.
 16. Extending prop according to claim 1,characterized in that the outer tube (2) and the inner tube (3) arearranged in the interior of a jacketing consisting of two tubes (23,24), the latter being plugged one into the other and guided on eachother in the way of a telescope, said jacketing having a greaterdiameter than the extending prop over its entire length, whereby theannular space between the extending prop (2, 3) and the jacketing (23,24) is fillable with construction material.